Comprehensive Analysis of Chemical Energy: Concepts and Applications

This document provides a detailed explanation of chemical energy, which is the energy released or absorbed during chemical reactions. It covers various aspects, including the storage of chemical energy in substances like batteries, food, and fuels. The document explains how energy changes are related to the breaking and making of chemical bonds, and the concepts of enthalpy and heat of combustion are discussed. It also explores the concept of chemical potential energy and how it relates to the structural arrangement of atoms and molecules. The transformation of chemical energy into other forms, such as heat and electrical energy, is also examined, along with the process of photosynthesis. The document references key scientific papers and the Wikipedia article on chemical energy to support the information provided.

Chemical energy
Chemical energy is the energy of chemical substances that is released when they undergo a chemical
reaction and transform into other substances. Some examples of storage media of chemical energy include
batteries,[1] food, gasoline, and oxygen gas.[2] Breaking and re-making of chemical bonds involves energy,
which may be either absorbed by or evolved from a chemical system.
Energy that can be released or absorbed because of a reaction between chemical substances is equal to the
difference between the energy content of the products and the reactants, if the initial and final temperature is
the same. This change in energy can be estimated from the bond energies of the reactants and products. It
can also be calculated from , the internal energy of formation of the reactant molecules, and
, the internal energy of formation of the product molecules. The internal energy change of a
chemical process is equal to the heat exchanged if it is measured under conditions of constant volume and
equal initial and final temperature, as in a closed container such as a bomb calorimeter. However, under
conditions of constant pressure, as in reactions in vessels open to the atmosphere, the measured heat change
is not always equal to the internal energy change, because pressure-volume work also releases or absorbs
energy. (The heat change at constant pressure is called the enthalpy change; in this case the enthalpy of
reaction, if initial and final temperatures are equal).
A related term is the heat of combustion, which is the energy mostly of the weak double bonds of
molecular oxygen[2] released due to a combustion reaction and often applied in the study of fuels. Food is
similar to hydrocarbon and carbohydrate fuels, and when it is oxidized to carbon dioxide and water, the
energy released is analogous to the heat of combustion (though assessed differently than for a hydrocarbon
fuel — see food energy).
Chemical potential energy is a form of potential energy related to the structural arrangement of atoms or
molecules. This arrangement may be the result of chemical bonds within a molecule or interactions between
them. Chemical energy of a chemical substance can be transformed to other forms of energy by a chemical
reaction. For example, when a fuel is burned, the chemical energy of molecular oxygen is converted to
heat.[2] Green plants transform solar energy to chemical energy (mostly of oxygen) through the process of
photosynthesis, and electrical energy can be converted to chemical energy and vice versa through
electrochemical reactions.
The similar term chemical potential is used to indicate the potential of a substance to undergo a change of
configuration, be it in the form of a chemical reaction, spatial transport, particle exchange with a reservoir,
etc. It is not a form of potential energy itself, but is more closely related to free energy. The confusion in
terminology arises from the fact that in other areas of physics not dominated by entropy, all potential energy
is available to do useful work and drives the system to spontaneously undergo changes of configuration,
and thus there is no distinction between "free" and "non-free" potential energy (hence the one word
"potential"). However, in systems of large entropy such as chemical systems, the total amount of energy
present (and conserved according to the first law of thermodynamics) of which this chemical potential
energy is a part, is separated from the amount of that energy — thermodynamic free energy (from which
chemical potential is derived) — which (appears to) drive the system forward spontaneously as the global
entropy increases (in accordance with the second law).
References

1. Schmidt-Rohr, K. (2018). "How Batteries Store and Release Energy: Explaining Basic
Electrochemistry", J. Chem. Educ. 95: 1801-1810.
http://dx.doi.org/10.1021/acs.jchemed.8b00479
2. Schmidt-Rohr, K. (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ
per Mole of O2", J. Chem. Educ. 92: 2094-2099.
http://dx.doi.org/10.1021/acs.jchemed.5b00333
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